Infrared sensors have a variety of applications. For example, infrared imagery has become an important part of climate studies, weather forecasting, severe storm tracking, meteorology research, geoscientific studies, and other applications. More particularly, infrared radiation from the Earth's surface and atmosphere can be detected using instruments carried by satellites to provide information regarding various phenomena. However, accurate calibration of such instruments is essential to detection accuracy. The evolution of remote sensing missions presents a growing need for satellite sensors with significantly enhanced measurement accuracies beyond current capabilities.
Infrared instruments are typically calibrated before they are deployed (i.e. prior to launch) to high degree of accuracy. However, the accuracy of such instruments will invariably degrade over time. Accordingly, periodic recalibration is required in order to maintain desired levels of accuracy. For example, the on-board calibration needed for very precise (e.g., <1% radiance uncertainty), spectrally resolved IR radiances typically requires the availability of high-emissivity (0.999) calibration blackbodies. Such high emissivity levels cannot be provided by conventional low emissivity or “black” surfaces. Indeed, even conventional “cavity blackbodies”, which combine low emissivity surfaces and relatively deep cavities, have coating emissivities that are usually limited to 0.98. To achieve emissivities of higher than 0.995 they employ complex geometries, with a depth that is typically more than double the aperture diameter, which results in a relatively large size and weight. The size and mass of cavity type blackbodies can in turn make changing the temperature of such devices time consuming and can require large amounts of power. Accordingly, high performance cavity type blackbodies are expensive to produce, and are challenging to accommodate in space systems with limited size, weight, and power (SWaP) allocations. Therefore, although conventional high performance blackbodies can meet the <1% radiance uncertainty requirement, their cost and SWaP impacts are large.
As an alternative to other high emissivity blackbodies, such as cavity blackbodies, blackbody surfaces comprising a planar surface of carbon nanotubes have been proposed and developed. These surfaces can be much smaller in the third dimension (i.e. in a dimension along a line of sight of an instrument being calibrated) than cavity type blackbodies. In addition, such surfaces can provide good performance over a limited area. However, they can suffer from variations in surface temperature and emissivity. These variations increase with the area of the surface. Moreover, the relatively low mass of such surfaces can make them more susceptible to temperature instability and non-uniformity. Accordingly, the uniformity of carbon nanotube type blackbody surfaces has been insufficient for use as a calibration surface for high performance instruments.